This invention relates to mass spectrometry and the measurement of the gain of an ion or particle detector.
In mass spectrometers, charged particles or ions are formed from molecules of a sample of interest and the mass-to-charge ratios of the ions are determined. In many instruments, these ions are ultimately detected by a detector system which contains electron or photo multipliers. In order to assure quantitative values of the number of ions and to optimize signal-to-noise ratios, the gain of the detector system must be known and often set to an optimum value.
The most direct way of measuring gain of a detector system is simply to measure the current going into the detector and the current coming out of the detector using a Faraday Cup or some other electrode. The gain is simply the ratio of the current out divided by the current in. Unfortunately, this technique is not practical in many mass spectrometers since it requires extra ion optical components which are complex, costly, and could hinder the performance of the system as a mass spectrometer. For this reason, other less invasive methods of determining the gain of the detector system are desired.
Many naturally occurring events occur at irregular, random, intervals such as radioactive decay, the arrival of photons from ordinary light sources, and the arrival of ions at a detector. The occurrence of these processes is characterized and controlled by the Poisson type of probability distribution. One consequence of this probability distribution is that the statistical fluctuation or variance of the measured ion intensity reaching the electron multiplier detector, under the appropriate conditions, is directly related to the average (or mean) number of ions detected. Based on this fact, one approach to measuring the gain of an electron multiplier has been described by Fies (International Journal of Mass Spectrometry and Ion Proceedings, 82 (1988) pp. 111–129 (incorporated herein by reference)).
The Fies method depends on taking multiple measurements of the intensity of a single type of ion in order to determine the number of ions measured. Once the number of ions is determined, a simple calculation using the known transfer function of the electronics can yield an ion detector's gain. This method, however, assumes that the variance of the measured ion intensity is solely due to the inherent variance of the ion beam, i.e. basic ion statistics, and that all other sources of irreproducibility are statistically negligible.
As depicted in FIG. 1, all mass spectrometers 100 include a source of ionization 110 which produces ions from a sample, ion transfer optics 120 to deliver the ions from the source to the mass analyzer 130, a mass analyzer 130, and some kind of ion detector system 140. Different types of chemical samples require different types of ionization techniques in order for the sample to be analysed by mass spectrometery. Operation of an Electron Ionization or Electron Impact (EI) source 200 (see FIG. 2) applied to volatile samples begins with the passing of current through wire(s) 210 to produce the electrons 220 that are subsequently used for the ionization process. The number of electrons emitted by the wire can be quite precisely controlled by adjusting the current passing through the wire in an electronic feedback loop based on sensing the emission current. Molecules 230 of the analyte 240 present in the gas phase are then passed through this electron beam, the molecules 230 are caused to lose an electron and analyte ions 250 are produced. The rate at which analyte molecules 230 pass through the electron beam can also be quite accurately controlled, and so an EI source 200 can be operated such that a relatively constant stream of analyte ions 260 is produced. Under these circumstances, the fluctuations in the ion beam intensity due to the stability of the ion source parameters is negligible and such sources therefore satisfy the assumption for the simple method described above. Other types of sources of ionization, including, but not limited to chemical ionization sources, also satisfy the assumption.
Mass spectrometry has seen a significant increase in its use for less volatile samples, including those in the condensed or liquid phase. This growth in applications is due to the development of atmospheric pressure ionization (API) techniques. An example of a mass spectrometer which incorporates an API source is shown in FIG. 3. Atmospheric Pressure Ionization sources (API) 300 are ion sources in which samples, typically in the condensed phase, such as liquids or solids, are ionized directly at atmospheric pressure, and are then transferred to the mass analyzer 395. The sample is typically dissolved in an appropriate solvent before being introduced into the mass analyzer 395 for analysis. The sample ions are transferred into the mass analyzer through a series of differentially pumped stages 310, 320, 330, 340, enabling a large pressure differential to be maintained between the API source 300 and the mass analyzer 395, without using unnecessarily large vacuum pumps.
ElecroSpray Ionization (ESI) is one type of API source. ESI occurs directly from solution at atmospheric pressure and provides highly charged droplets of the solution. In ESI, a capillary or needle has its orifice in close proximity to the entrance into the vacuum system of the mass spectrometer, and a dilute solution, containing the sample molecules of interest, is pumped through the needle. A strong potential, typically 1–5 KV is applied between the needle orifice and an orifice leading to the mass analyzer. This forms a fine “spray” of the liquid solution. The spray of droplets evaporates to produce ions of the sample, and a mixture of ions, droplets and neutral particles enter the mass analyzer via the orifice.
In electrospray ionization, the quality of the mass spectrum is strongly dependent on the quality of the spray emitting from the needle, i.e. on its fineness and its consistency. Since the quality and stability of the spray are strongly dependant on the electric field which in turn is dependent on the mechanical nature of the needle, and also on the liquid flow properties at the tip, stability of the rate of generating ions is often problematic. The quality of the spray can somewhat be determined by direct visualization of the spray and also by monitoring the current emitted from the needle. Some sources also utilize a strong flow of gas to assist in nebulizing liquid samples and to further help break down the solvent droplets. The liquid characteristics, such as viscosity and ionic strength, and the gas characteristics, such as temperature and flow rate, all have an effect on the stability of droplet production and the electrospray process.
Consequently, due to the many parameters involved and to the nature of generating a liquid spray, the stability of the spray, and therefore the production of ions is not stable enough to be neglected compared to the inherent variance of the ion beam, even under optimum conditions. In this type of ion source, there is a temporal instability inherent in the nature of the source. This temporal instability can dominate the observed ion intensity variance and render invalid the assumptions on which methods such as the single ion Poisson statistics method or Fies method depend.
In instruments in which the ion beam from the source is continuously being detected, so called “beam machines”, a particular type of ion or single ion m/z can be chosen to be continuously passed to the detector. These types of instruments do not have to scan over a mass spectral peak but can be parked on top (or the side) of a spectral peak and intensity measurements can be made continuously. One consequence of this is that if a measurement of the intensity of another mass is required, this measurement occurs on a different part of the ion beam from the source at a different time. If the source of the ions is unstable, the temporal variation in the ion beam could severely affect the apparent ratio or difference of the intensities of the two different ions.
In instruments in which a fraction or packet of the ions produced in the source is sampled or integrated and then analyzed, there is a time gap between the sampling of the ion beam and measuring their intensities. Thus, a true continuous measurement is not practical. Examples of these “pulsed” or “trapping” types of mass analyzers include ion trap mass spectrometers (both 3D and 2D linear traps, Fourier Transform mass spectrometers, Orbitrap analyzers, and time of flight mass spectrometers). In trying to make continuous ion current measurements, some of these instruments can be put into a transmission mode in which the ions from the source are attempted to be continuously transferred to the detector. However, in this mode no mass analysis is possible and the actual identity of the ions which reach the detector is unknown. Since the actual gain of the detector can depend on the actual ion species which it is detecting, it is desirable to know the identity of the ions for which the gain is measured. Utilization of the same ion to both determine and set the gain of the detector provides consistent results on different instruments, even with different ionization sources.
In the basic method described by Fies, the effects of various sources of error and how to possibly identify them in the results is discussed. The types of errors considered include errors due to bandwidth, noise spikes (voltage spikes induced in the electronic circuits of instruments from outside sources, such as nearby electrical machinery, other poorly shielded instruments, etc.), errors due to zeroing of the electrometer, amplifier noise in excess of shot noise (e.g., coherent noise such as power line-related ripple), digitizing errors (round-off and finite dynamic range), errors due to peak modulation and errors due to electron multiplier noise. Although Fies discloses ways to possibly identify these sources of error, he does not consider methods for eliminating the effects of these noise sources on the results in order to accurately determine the gain. In fact, Fies states clearly that his technique for determining gain depends on the assumption that the system “is free of all noise except for the statistical fluctuations due to the entering ion beam”. This invention describes how to do this with respect to at least one specific source of noise, namely ion source instability, but also would apply to other sources of noise which have common mode effects on the different ion intensities.